We are studying the mechanisms of cleavage plane specification in early embryos of the nematode Caenorhabditis elegans. Two basic types of cell division are observed during embryonic development: proliferative divisions in which both daughter cells have the same developmental potential and determinative divisions where the developmental potential of the daughters differ. The pattern of cell cleavages is different for proliferative and determinative divisions: the cleavage planes of proliferation cells are always orthogonal with respect to the cleavage plane that produced the mother, in contrast, the early determinative cleavages in. elegans are all in the same transverse plane. Furthermore, determinative division are often asymmetric with different sized daughters, whereas proliferate divisions are always symmetrical. A genetic screen capable of isolating a large collection of conditional cell division mutants has been devised. To date, 29 mutant lines that meet these criteria have been isolated. We find that the mutants exhibit a wide variety of phenotypes with respect to their pattern of cell division. We are making extensive use of the software developed at the INM for 4D data collection and analysis. We are using this software to make multiple focal plane, time-lapse recordings of embryonic development in order to characterize mutant phenotypes. In addition, we are studying the organization of the cytoskeleton of mutant embryos using immunofluorescence. We are using the multiphoton imaging system because of its ability to image UV excited fluorophores simultaneously with green and blue excited fluorophores. we are also planning to use the multiphoton system for in vivo imaging of fluorophore-derivatized tubutin, so that we can study in vivo microtubule dynamics in mutants with a misplaced mitotic apparatus.